Dielectric actuators in the form of laminates (or sandwich structures or layered materials) belong to a rapidly evolving field of technology. A simple dielectric actuator may comprise a layer of electroactive polymer (EAP) and an electrode pair for applying an electrostatic force and thereby causing an elastic deformation of the polymer layer in the tangential (in-plane) and/or transversal (out-of-plane) direction. More sophisticated dielectric actuators may include additional functional layers, such as optically reflective or antireflective layers, texture-enhancing layers, electrically and/or thermally conductive layers, etc.
The way in which a dielectric actuator responds to an applied electric field may be influenced by the addition of passive layers. The application published as U.S. 2008/0289952 discloses an actuator coated with one or more passive polymer layers. The passive layers respond indirectly to variations in the actuating field under the action of shearing forces exerted on them by the actuator. Thus, as shown in FIG. 1 of the present application, expansion of an active area D between the electrodes E1, E2 of the actuator stretches the passive layers PL1, PL2 so that an elevated edge, corresponding to the boundary of the active area D, is produced on an external surface TDS of the laminate TDU. (It is noted that the visible difference in size of the electrodes E1, E2 is not a feature common to all actuators of this type.) To further illustrate such stretch mode movement, FIG. 2 shows how compression—and accompanying planar stretching—of that portion of an EAP layer 202 which is located between two electrodes 210, 211 causes an amplified thickness contraction of surrounding passive layers 210, 211 by the Poisson effect. The material making up the elevated edge is supplied from the active area by stretching the latter into a thinner shape; such stretching may not be acceptable in all applications.
Koo, Jung et al., Development of soft-actuator-based wearable tactile display, IEEE Trans. Robotics, vol. 24, no. 3 (June 2008), pp. 549-558 discloses a dielectric actuator, a portion of which is capable of buckling movement, as shown by FIG. 3 of the present application. The active portion 320 of the actuator 302, 310, 311 is clamped inside a rigid boundary frame (not shown), thus not in elastic contact with the surrounding portion 321. The clamping restricts tangential expansion and causes the actuator to deflect out-of-plane instead, a preferred direction of buckling being defined by the presence of a passive layer 301. While actuators of this type may achieve a relatively large deflection amplitude, they are generally unable to produce sharp edges, hence not ideal for tactile applications. Further, it has turned out that buckling-mode actuators perform best for symmetric shapes, such as square-shaped or round shapes, and will therefore not be compatible with too irregular electrode shapes.